auxdisplay: charlcd: checking for pointer reference before dereferencing

Check if the pointer lcd->ops->init_display exists before dereferencing it.
If a driver called charlcd_init() without defining the ops, this would
return segmentation fault, as happened to me when implementing a charlcd
driver.  Checking the pointer before dereferencing protects from
segmentation fault.
Signed-off-by: Luiz Sampaio <sampaio.ime@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

drivers/auxdisplay/charlcd.c

@@ -578,6 +578,9 @@ static int charlcd_init(struct charlcd *lcd)
 	 * Since charlcd_init_display() needs to write data, we have to
 	 * enable mark the LCD initialized just before.
 	 */
+	if (WARN_ON(!lcd->ops->init_display))
+		return -EINVAL;
+
 	ret = lcd->ops->init_display(lcd);
 	if (ret)
 		return ret;